Why does parenting Heel IK to Thigh Bone cause jittering in right leg but not left leg?
To have an IK controller which is child of a bone which is part of the IK chain must be considered a rigging error, because it creates a circular dedendency, leading to unpredictable behaviours (and jitterings).

Instead you can create a new bone (which I called "scaler") that will be parent of the thigh and the HeelIK. In my picture scaling the scaler bone caused the whole left leg to scale proportionally, maintaining full functionality of the IK leg chain, without any need of constraints.